A spark plug including a pre-chamber for a combustion chamber of an engine is known. For example, see Japanese Unexamined Patent Application Publication No. 2006-144648 (“PTL 1). This type of spark plug includes a plug cap that is connected to a metal shell and that has a through hole. The plug cap is exposed in the combustion chamber so that the pre-chamber is provided in the combustion chamber. Combustible air-fuel mixture flows into the plug cap from the combustion chamber through the through hole. The spark plug ignites the combustible air-fuel mixture that has reached a spark gap, so that the combustible air-fuel mixture is combusted to generate an expansion pressure that causes a gas flow including flame to be injected into the combustion chamber through the through hole. The combustible air-fuel mixture in the combustion chamber is combusted by the injected flow of flame.
However, according to the technology disclosed in PTL 1, when variation in disruption of the combustible air-fuel mixture that has flowed into the plug cap from the combustion chamber through the through hole increases, the combustible air-fuel mixture does not easily reach the spark gap as designed. Accordingly, growth of a flame kernel generated in the spark gap may be suppressed and the combustion stability may be reduced.